The present invention relates to an anto-focus system for a video camera, or more in particular to a system having an auto-focus function not only for photographing over a normal distance range but also for a close-up photo mode as well.
Conventional auto-focus systems for a video camera measure a distance between the camera and an object to be photographed on the principle of triangulation such as a double-image matching system and move an objective lens to a predetermined position by a motor in a manner to focus the objective lens to the measured distance.
Such systems or systems similar thereto have a sufficient auto-focus function over the normal photographing distance range of about 1 mm to .infin. from the video camera to the object. However, when the distance is less than 1 m, particularly, in close-up photography or what is called macroscopic photography in which the distance is only several cm, auto-focus is impossible.
The auto-focus system of the triangulation type is inoperable for macroscopic photography mainly for following three reasons.
(1) In the triangulation type of focusing system or a system similar thereto, distance measurement of high accuracy is impossible over the large region from several cm to infinity. The triangulation type of focusing system measures the distance such that the deviation angle of a measuring mirror is made to correspond to the distance. Measurement of the distance in a range of several cm to infinity requires the mirror to deviate by an angle of several tens degrees, and it is not particularly feasible to deviate the mirror by such a large angle with high accuracy. PA0 (2) When the distance between an object and the camera is about several cm, the parallax between the triangulation unit and the object lens becomes large, resulting in an increased probability that the measured distance does not always represent the actual distance from the camera to the object to the photographed. PA0 (3) In the conventional auto-focus systems, the focusing operation is attained by controlling the rotation of a distance ring for the object lens. Since the focusing operation for the macroscopic range usually uses a zoom ring, however, macroscopic photography requires stopping the operation of the auto-focus system and adjusting the zoom ring manually for focusing.
These problems seem to be able to be solved by using such a system for correcting a mirror deviation angle with high accuracy, a system for correcting parallax, or a system for applying an auto-focus signal to the zoom ring instead of to the object lens for macroscopic photographing. In the present state of art, however, it is impossible to realize such systems. For the present auto-focus systems, therefore, a suggestion has been made to stop the auto-focus function at the time of macroscopic photographing.
Another example of the conventional auto-focus systems for obviating the above-mentioned disadvantages (1) and (2) of the triangulation type is disclosed in U.S. Pat. No. 4,320,417 entitled "Automatic Focusing System for Video Camera".
This prior art teaches the technique of passing the video signal from a video camera through a high-pass filter to obtain a high frequency component, sampling a voltage proportional to the fineness or extent of freedom from confusion of the television image (hereinafter referred to as focus voltage) by detecting the extracted high frequency component and bringing a focusing ring (hereinafter referred to as helicoid) to a position at which the focus voltage is maximum, i.e., a position of focus, by taking the difference between two consecutive sampled values and controlling the rotation of the helicoid by a reversible motor such that the difference is always positive.
Further, in order to prevent error in the measuring operation of the distance caused by the movement of the object being photographed or the panning of the camera, a movable range of the master lens is limited in accordance with the focal length thereof.
The above-mentioned U.S. Patent does not describe photographing in the macroscopic region.